In the early twentieth century, scientists commonly viewed self-experimentation an essential part of medical research. Self-exposure to untested interventions was believed the most ethical way to assess human responses to those interventions, and to catalyse further research (Dresser 2013). Some of this research helped to found new scientific fields. Respiratory physiology was one such field, formed in the 1920s through self-experiments conducted by scientist John Haldane and colleagues. In 1984, physician Barry Marshall ingested Helicobacter pylori, which helped to establish the link between H. pylori and gastric pathology, and in 1992, self-experiments conducted by Mike Stroud and Ranulph Fiennes in Antarctica advanced understanding of nutrition in extreme conditions.

Self-experiments to study pain experience have been published by Sir Head (1920), Woollard and Carmichael (1933), Landau and Bishop (1953), Price (1972), Price et al. (1977), and Staud et al. (2001, 2008), to name only a few significant investigator-participants who studied pain. William Landau and George H. Bishop conducted standard psychophysical research on themselves to study the qualitative differences between “first pain” and “second pain” (i.e. “double pain”; later termed epicritic and protopathic pain) (Landau and Bishop 1953). Initially, Landau and Bishop identified through introspection the differential experiential qualities between first and second pain, followed by scientifically informed speculation about the mechanistic difference between the two types of pain. They discovered that first pain was sharp or stinging, well localized, and brief, whereas second pain was dull, aching, throbbing, or burning, and poorly localized, and longer lasting. The qualities of second pain were felt when skin C-nociceptors were stimulated.

These findings were subsequently confirmed by Price (1972) based on researcher and naïve participant introspective reports. Temporal differences between first and second pain were introspected on and mechanistically explained in terms of central temporal summation in studies by Price et al. (1977), and Staud et al. (2001, 2008), using investigator- and naïve-participants.

Conducting self-experiments to study referred pain, collaborators Herbert Woollard and Edward Carmichael observed that 300 g of weight placed on the right testicle produced slight discomfort in the right groin, while 650 g on the right testicle caused severe pain on the right side of the body. They confirmed that injury to the testicles caused pain to be referred throughout the body. For instance, as the weight on the testicle increased to over 900 g, they reported pain “of a sickening character” not only in the groin but also spreading across the back (Woollard and Carmichael 1933).

Self-experimentation on pain has on occasion led to surprising results. The psychologist B. Berthold Wolff self-experimented in his pain psychophysics laboratory, varying thermal pain which was produced at that time by briefly shining a strong light on a spot on the forearm blackened with candle black for a calibrated time and intensity of exposure (Hardy et al. 1940). On one occasion, Wolff pushed the button to deliver the noxious stimulus, but then something unexpected happened: he screamed with pain, which was brief but intense and filled his whole body. He described it as the most intense whole-body pain he had ever experienced. Wolff later discovered that the light stimulus had been knocked off its correct aim, and had missed his forearm altogether and instead diffused onto the opposite wall where it created a very strong flash of light throughout the normally dark room. Wolff speculated that, as he was expecting to feel pain, the unexpected flash of strong light had the same effect, producing an experience of pain.

It is unclear if investigators today independently conduct self-experiments or co-participate in their own pain studies. The convenience of recruiting participants from university classes and the internet may have made self-experimentation or co-participation of pain seem somewhat redundant to researchers. The Declaration of Helsinki advises on conducting ethical research using patients and healthy volunteers, although it is unclear if this is reason enough for challenging independent self-experimentation or investigator co-participation. In self-experiments, the researcher is both investigator and single participant, so the requirement for informed consent could be waived. Still, there is clear historical precedent for scientific investigators successfully observing and analyzing their own experiences of pain. The results of such published self-experiments have been integrated into the body of knowledge of pain, and replicated in numerous studies using naïve participant introspective reports and standard scientific methods.

The question of investigators introspecting on their own personal pain experiences in pain studies has received little attention in the literature. Study of this question may reflect ethical reservations about the many points at which self-interest may lead us to introspect on personal experiences through personal biases that in turn impair professional decision-making and perception. Despite this valid concern about research co-participation, we offer three reasons why investigators can introspect on personal pain as co-participants in their own pain studies. First, there is historical precedent for investigator participation and co-participation in scientific pain research using introspection as a study method. Second, general concerns about variability in self-report based on introspection on pain experience partly derive from true fluctuations in personal pain experience and perceived interests in self-reporting pain, not simply error in its scientific measurement. Third, the availability of the Experiential-Phenomenological Method, a mixed research method for the study of human experiences, allows investigators to co-participate with naïve participants in their own studies by encouraging passive introspection on personal pain experiences.

Introduction: The definition of pain promulgated by the International Association for the Study of Pain (IASP) is widely accepted as a pragmatic characterisation of that human experience. Although the Notes that accompany it characterise pain as “always subjective,” the IASP definition itself fails to sufficiently integrate phenomenological aspects of pain.

Methods: This essay reviews the historical development of the IASP definition, and the commentaries and suggested modificationsto it over almost 40 years. Common factors of pain experience identified in phenomenological studies are described, together with theoretical insights from philosophy and biology.

Results: A fuller understanding of the pain experience and of the clinical care of those experiencing pain is achievable through greater attention to the phenomenology of pain, the social “intersubjective space” in which pain occurs, and the limitations of language.

Conclusion: Based on these results, a revised definition of pain is offered: Pain is a mutually recognizable somatic experience that reflects a person’s apprehension of threat to their bodily or existential integrity.